1. Field of the Invention
The present invention relates to a muffler that is used in an exhaust system of a combustion engine mounted on a motor vehicle or the like and is equipped with an inner shell portion and an outer shell portion wrapping an outer surface of the inner shell portion.
2. Description of the Related Art
A muffler of this kind is disclosed in Japanese patent laying-open publication No. 2002-206422. This muffler has a shell main body made of a double-rolled flat plate or triple-rolled flat plate, forming an inner shell portion and an outer shell portion, where their overlapped portions are spot-welded together. This shell main body is formed at each end portion with an opening portion, which is fixed by an end plate so that its opening is closed.
Such a conventional muffler is also shown in FIGS. 7 and 8. The muffler includes a shell main body 01 and two end plates 02 and 02. The shell main body 01 is manufactured by double-rolling a flat plate to have two layers forming an inner shell portion 04 and an outer shell portion 03 wrapping an outer surface of the inner shell portion 04 so that transmitting noise from an interior of the muffler can be reduced. The outer and inner shell portions 03 and 04 are partially overlapped to form overlapped portions 05, which are spot-welded together at several positions indicated by 06. Two end plate 02 shaped like a dish are press-fit into openings of the shell main body 01, respectively, and then an outer peripheral portion of the end plates 02 and the end portions of the outer shell portion 03 are laser-welded or metal-insert-gas-welded along their outer circumferences 07 to fix with each other.
The conventional mufflers, however, encounter a problem in that the end plates 02 can not be smoothly inserted into the openings of the shell main body 01 before the end plates 02 are press-fit into the openings, because the outer peripheral portion of the end plate 02 sometimes interferes with an opening side end portion of the inner shell portion 04, which is easily twisted and projected inwardly due to residual stress caused when the opening side end portion, to be the inner shell portion 04, of the flat plate is inserted into a groove of a jig to be clamped and the flat plate is rolled around the jig to form the shell main body 01. The unsmooth insertion and interference of the end plates 02 cause trouble in arrangement thereof before press-fitting, decreasing productivity rate of the mufflers.